1. Field of the Invention
The present invention relates to a heat-treatment furnace.
2. Description of the Related Art
In a process for manufacturing a semiconductor device, wafers as objects to be treated are carried into a heat-treatment furnace, such as a horizontal or vertical furnace, to be subjected to heat treatment therein. The heat-treatment furnace of this type comprises a process tube for containing process gas, an electrically-operated heater disposed around the process tube, and an adiabatic member covering the heater.
Conventionally, the process tube is of a batch type such that a large number of wafers are contained in the tube so as to be arranged in the longitudinal direction of the tube. Accordingly, the region for containing the wafers must be made to be an isothermal zone which is kept at a constant temperature. In order to maintain the uniformity of the treatment for the wafers, more specifically, the range of temperature fluctuations in the isothermal zone must be restricted within plus or minus 0.5.degree. C.
In the conventional heat-treatment furnace, therefore, the heater is divided, for example, into three or five temperature control zones, along the longitudinal direction of the process tube. In this case, a temperature profile along the longitudinal direction of the process tube is uneven, as indicated by characteristic curve I in FIG. 4. As a measure for improving such an uneven temperature profile, a soaking tube formed of SiC or the like is conventionally interposed between the process tube and the heater, besides the aforesaid temperature control system (Published Unexamined Japanese Patent Application No. 60-148124).
In a heat-treatment furnace with the soaking tube, however, one end portion of the process tube is opened to serve as a furnace opening through which the wafers are carried in and out, so that the opening portion is located in a temperature gradient zone even though the opening is closed during the treatment. Thus, this temperature profile is subject to a problem such that heat escapes from the furnace through the furnace opening portion, thereby entailing a temperature drop, as indicated by characteristic curve II in FIG. 5. In order to prevent such heat radiation, the heating temperature of heater elements on the side closer to the open end of the process tube is set relatively high. If the heating temperature of specific heater elements is increased, however, the temperature of the central portion of the process tube fluctuates, so that the isothermal zone is shortened. As a result, batch-type heat treatment will produce plenty of defects.
Even though the soaking tube of SiC or the like is interposed between the heater and the process tube, moreover, it cannot be very conducive to the prevention of dispersion of the temperature profile on the open end side of the isothermal zone. The soaking tube is better serviceable for the prevention of contamination than for the maintenance of the isothermal zone length. More specifically, dust from the adiabatic member, heater, etc. enters the process tube through its wall, thereby lowering the yield of treatment. With use of the soaking tube disposed outside the process tube, however, the amount of dust transmitted through the tube can be reduced. Thus, the conventional soaking tube cannot effectively prevent heat from escaping from the heater toward the open end of the furnace.